Project Summary Fear extinction, the process of learning that cues associated with past trauma no longer pose a threat, is a common target of therapeutic strategies for common anxiety and trauma-related disorders. However, fear extinction memories are labile, and fear and anxiety tend to relapse even following successful extinction. Unfortunately, manipulations of known extinction circuits have had limited success reducing relapse. We have observed that activation of the nigrostriatal dopamine (DA) pathway, consisting of DA neurons in the substantia nigra (SN) that project to the dorsal striatum (DS), during fear extinction can enhance extinction and reduce relapse. Since the nigrostriatal DA pathway has not previously been considered in the context of fear extinction, understanding how nigrostriatal DA reduces relapse could lead to novel strategies for the prevention of relapse after extinction. Our published & preliminary data indicate that sub-circuits within the nigrostriatal DA pathway that target distinct medial (DMS) & lateral (DLS) networks of neurons in the DS have unique roles in fear extinction & relapse. SN DA neurons projecting to the DMS (DASN-DMS circuit), but not SN DA neurons projecting to the DLS (DASN-DLS circuit), seem to contribute to normal fear extinction that remains susceptible to relapse. However, in order for activation of the nigrostriatal DA pathway to reduce relapse, the DASN-DLS circuit needs to be recruited during fear extinction training & retrieval. Consistent with this possibility are exciting emerging data in females challenging existing dogma that deficits in fear inhibition contribute to the higher prevalence of trauma- related disorders in females than males. We have observed that female rats can be protected from relapse if exposed to fear extinction training during phases of the estrous cycle when estrogen is high. Recruitment of the DASN-DLS circuit could contribute to this sex difference, as preliminary data indicate that females exposed to fear extinction when estrogen is high recruit both the DMS & DLS, whereas extinction normally recruits the DMS, but not the DLS. Together, these data support the hypothesis that the DASN-DMS circuit supports fear extinction & can strengthen fear extinction retrieval in males & females with minimal impact on relapse. In contrast, the DASN-DLS circuit is not normally involved in fear extinction, but can render fear extinction memory resistant to relapse if recruited during fear extinction training, such as occurs in females during estrous phases with high estrogen. In Aim 1, we will determine the DASN-DS sub-circuits involved in fear extinction in males and cycling females by 1) measuring their activity during fear extinction using dual-viral neural tract tracing and 2) inhibiting their activity using retrograde chemogenetics. In Aim 2, we will use retrograde optogenetics to determine if augmentation of fear extinction & protection from relapse can be separately produced by activation of unique DASN-DS sub-circuits. We will also determine if the DLS is necessary for retrieval of a relapse-resistant extinction memory produced by either optogenetic activation of the DASN-DLS circuit or learning fear extinction when estrogen is high.